1. Field of the Invention
This invention relates to a method and apparatus for adjusting certain physical dimensions of the view finding system of a single-lens reflex camera to permit a clear and sharp image to appear on the focusing screen of the camera.
More specifically, this invention relates to such an adjusting method and apparatus in which the focusing adjustment of the view finder system is carried out by reducing the deviation from a slant angle of 45.degree. of the swingable mirror and by translating the swingable mirror toward or away from the focusing screen to thereby properly adjust the view finder system without need of adjusting the focusing screen.
2. Description of the Prior Art
As shown in FIG. 1, a single-lens reflex camera has a mirror box 2 in a camera body 1. A swingable mirror 4 is fitted in a mirror frame 3. The mirror frame is rotatably fixed to the mirror box 2 in such a way that the swingable mirror 4 is capable of swinging upward about a shaft 5. An actuating mechanism (not shown) for swinging up the mirror 4 when the shutter button is depressed is provided on one side of the mirror box 2. A focusing screen 6 and an associated condenser lens 7 are fixed in place on a shelf 2a at the top of the mirror box 2. Other important parts of the single-lens reflex camera are a taking lens 8 fixed to rest flush on an annular plane 1a of the camera body 1, a photosensitive film 9, a penta prism 10 and an exposure meter 11 having a pointer 11a.
With this arrangement, a high degree of dimensional precision is required in several respects. First it is essential that the plane 1a on which the taking lens is supported should lie exactly parallel to the plane 1b on which the photosensitive film is supported. Again the shelf 2a on which the focusing screen 6 rests must be precisely parallel to the optical axis "OL" of the taking lens. Furthermore, the point "P" at which the beam of light traveling along the optical axis "OL" falls on and reflects from the swingable mirror 4 must be exactly the same distance from both the plane 1b and the plane 1c. (That is to say the optical distance between plane 1a and the focal plane 6a of the focusing screen 6 via the point "P" must be equal to the optical distance between the plane 1a and the plane 1b via the point "P" or, in other words, the relationship a + b = a + c must be satisfied where "a" stands for the distance from the reflecting point "P" to the plane 1a; "b" stands for the distance from "P" to the plane 1b; and "c" stands for the distance from "P" to the focal plane 6a of the focusing screen.)
Ordinarily, an attempt is made to attain a satisfactory degree of precision in these respects by a method that will now be described with respect to FIG. 1. First, one or the other of the planes 1a and 1b is machined and is then used as the standard for trimming the other plane. This is a fairly simple process by which a high and quite satisfactory degree of parallelism can be attained between the two planes without much difficulty. Next the flat surface (1c in FIG. 1) of the bracket on which the mirror box is fixed within the camera body is trimmed with reference to the planes 1a and 1b already machined. That is to say, the bracket surface 1c must be trimmed so as to be exactly perpendicular to the planes 1a and 1b and, furthermore, so as to be a precise distance "d" (determined by the thickness of the mirror box 2 and other factors) from the optical axis "OL". In this process, it is very difficult to attain a satisfactory degree of precision even with the most expensive equipment and skilled workmanship. Where there is a restriction on the production cost of the camera, the degree of precision actually attained is invariably less than what is required. Setting of the swingable mirror at an angle of precisely 45.degree. presents another virtually insurmountable problem since this adjustment entails making compensation for various unknown factors such as the trueness of the mirror frame 3 and the shaft 5.
For practical purposes then, it is impossible to set the focusing screen with its focal plane exactly parallel to the optical axis and separated therefrom by precisely the distance "c" as defined earlier. In view of this impossibility, it is the ordinary practice to first set the swingable mirror to as close to 45.degree. as is possible and then to carry out the focusing adjustment by adjusting the focusing screen to reduce the overall error in the focusing system of the view finder. More specifically, this adjustment of the focusing screen is carried out by inserting an adjusting shim or shims in the space between the focusing screen 6 and the shelf 2a. In general, the maladjustment of the focusing screen 6 is not only one of improper distance from the optical axis "OL" but a combination of improper distance and non-parallelism resulting from the accumulated affect of errors appearing at different parts of the structure. Therefore, flat shims are useless and a variety of adjusting shims are required. Selection of a proper shim from among numerous shims prepared in advance is tedious and time-consuming work and is the cause for increased cost. In an attempt to obviate the need for using non-flat adjusting shims there has been proposed a structure as shown in FIG. 2 and described in Japanese utility model publication No. 8465/1957 which permits a final adjustment of the swingable mirror after the height of the focusing screen has been adjusted by the use of flat shims. As seen in FIG. 2, the proposed structure is provided with an elbow lever 51 rotatably fixed to one side or the other of the mirror box. The vertical extension of the elbow 51 has an arcuate slot 54 which extends along an arc of a circle having a pin 53 as its center. The swingable mirror 55 is pivotably connected via a pin 52 to the free end of the horizontal extension of the elbow. A crosswise deviation of the mirror 55 from the proper position can be reduced by rotating the elbow 51 about the pin 53 by an appropriate amount. Thanks to this adjustable structure, the shims pushed under the combined condenser lens and focusing screen 56 can all be made flat and, although they must be prepared in different thicknesses, there is no need for preparing non-flat shims of various shapes.
Another adjusting technique is disclosed in Japanese utility model publication No. 35319/1974 which relates to an improvement in the method of adjusting the swingable mirror to a slant of 45.degree.. Whereas this adjustment of the angle between the swingable mirror and the optical axis of the taking lens was previously performed at two selective points of the mirror catching-and releasing part so that the total adjustment would put the swingable mirror in the proper position, this utility model publication proposes a technique for reducing the number of adjustment points to only one, thereby reducing the difficulty encountered in setting the swingable mirror at 45.degree..
The two adjusting techniques mentioned above have one thing in common. Namely, in each the overall adjustment of the view finder system of the single-lens reflex camera is accomplished by separately adjusting two different elements of the system. That is, one part of the adjustment relates to the swingable mirror and is aimed at properly setting its angle with respect to the optical axis of the taking lens and the other part of the adjustment relates to the focusing screen and is directed to adjusting the distance between said screen and the optical axis. In this sense, these techniques are well-founded and useful. In analyzing the methods they employ, it might be said that in each case a certain part of the adjustment is shifted from the focusing screen to the swingable mirror and that, as a consequence, only flat shims are required thus obviating the necessity for first preparing and then carefully selecting and using shims of different shapes which would otherwise be indispensable in making the adjustment.
However, it is apparent that it would be still more advantageous to be able to perform the whole adjustment only by adjusting the swingable mirror. That is to say it would be most convenient to have a method and apparatus whereby what has heretofore been accomplished by separately adjusting the focusing screen and the swingable mirror could be attained solely by adjusting the swingable mirror thus obviating the need to adjust the focusing screen altogether.